Total transthoracic impedance (TTI) consists of internal thoracic impedance (ITI) and skin contact impedance. The TTI may vary from 920 to 1230 ohm in different individuals (11-13). However, change of TTI in PLE was reported by others to vary in the range of 2-16 ohm (7-9, 11-18), which is approximately1% of TTI and 1.5% of skin contact impedance. Monitors used in previous works were not sensitive enough to detect the relatively small changes in pulmonary impedance(2-7).